Add rclpy message lost status event demo (#457)

Signed-off-by: Ivan Santiago Paunovic <[email protected]>

Author: ivanpauno

quality_of_service_demo/README.md

@@ -161,6 +161,7 @@ export RMW_IMPLEMENTATION = rmw_connext_cpp
 ros2 run quality_of_service_demo_cpp message_lost_listener &
 # -s allows specifying the message size in KiB. This argument is optional, defaults to 8192KiB.
 ros2 run quality_of_service_demo_cpp message_lost_talker -s 8192
+# ros2 run quality_of_service_demo_py message_lost_talker -s 8192 to run the python listener.
 ```
 
 Example output:

quality_of_service_demo/rclpy/quality_of_service_demo_py/message_lost_listener.py

@@ -0,0 +1,76 @@
+# Copyright 2020 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import rclpy
+from rclpy.executors import SingleThreadedExecutor
+from rclpy.node import Node
+from rclpy.qos_event import SubscriptionEventCallbacks
+from rclpy.time import Time
+
+from sensor_msgs.msg import Image
+
+
+class MessageLostListener(Node):
+    """Listener node to demonstrate how to get a notification on lost messages."""
+
+    def __init__(self):
+        """Create a MessageLostListener."""
+        super().__init__('message_lost_listener')
+
+        # Create an object describing the event handlers that will
+        # be registered in the subscription.
+        # In this case, only a handler for a message lost event is registered.
+        event_callbacks = SubscriptionEventCallbacks(
+            message_lost=self._message_lost_event_callback)
+        # Create a subscription, passing the previously created event handlers.
+        self.subscription = self.create_subscription(
+            Image,
+            'message_lost_chatter',
+            self._message_callback,
+            1,
+            event_callbacks=event_callbacks)
+
+    def _message_callback(self, message):
+        """Log when a message is received."""
+        now = self.get_clock().now()
+        diff = now - Time.from_msg(message.header.stamp)
+        self.get_logger().info(
+            f'I heard an Image. Message single trip latency: [{diff.nanoseconds}]\n---')
+
+    def _message_lost_event_callback(self, message_lost_status):
+        """Log the number of lost messages when the event is triggered."""
+        self.get_logger().info(
+            'Some messages were lost:\n>\tNumber of new lost messages: '
+            f'{message_lost_status.total_count_change}'
+            f' \n>\tTotal number of messages lost: {message_lost_status.total_count}',
+        )
+
+
+def main():
+    rclpy.init(args=None)
+
+    listener = MessageLostListener()
+    executor = SingleThreadedExecutor()
+    executor.add_node(listener)
+
+    try:
+        executor.spin()
+    except KeyboardInterrupt:
+        pass
+    finally:
+        rclpy.shutdown()
+
+
+if __name__ == '__main__':
+    main()

quality_of_service_demo/rclpy/setup.py

@@ -27,10 +27,11 @@
     tests_require=['pytest'],
     entry_points={
         'console_scripts': [
-            'lifespan = quality_of_service_demo_py.lifespan:main',
-            'liveliness = quality_of_service_demo_py.liveliness:main',
             'deadline = quality_of_service_demo_py.deadline:main',
             'incompatible_qos = quality_of_service_demo_py.incompatible_qos:main',
+            'lifespan = quality_of_service_demo_py.lifespan:main',
+            'liveliness = quality_of_service_demo_py.liveliness:main',
+            'message_lost_listener = quality_of_service_demo_py.message_lost_listener:main',
         ],
     },
 )